The Nemeth Code tells us to format subdivisions in 3-5 (§191.b.ii). [The "yellow book" Introduction to Braille Mathematics did say in its section 69.b, "It is permissible to place the subdivisions of itemized material side by side across a page if all the subdivisions can be accommodated on one braille line."]
But we are talking about using only UEB (i.e., transcribing in UEB Math/Science), so for formatting questions we should look to BANA's Provisional Guidance for Transcribing Mathematics in UEB and then to Braille Formats 2016. The former does say,
Treat an exercise with subentries as a nested list and the indentation pattern is determined by the complete set, not individually. For example, if two levels: 1-5, 3-5; if three levels: 1-7, 3-7, 5-7, and so forth.
Braille Formats' section 10.4.4 talks about listing all answer choices vertically and being consistent in the method used for vertically listing items that are printed horizontally.
You are right that it is best practice to put each problem on its own braille line, even when they are printed horizontally across a line or lines. As far as we understand, having each problem on its own line makes the material most easily navigable for a braille user.